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<div class="section">
<div class="titlepage"><div><div><h4 class="title">
<a name="spirit.karma.tutorials.karma_easier_complex"></a><a class="link" href="karma_easier_complex.html" title="Complex - Made easier">Complex
        - Made easier</a>
</h4></div></div></div>
<p>
          In the previous section we showed how to format a complex number (i.e.
          a pair of doubles). In this section we will build on this example with
          the goal to avoid using semantic actions in the format specification. Let's
          have a look at the resulting code first, trying to understand it afterwards
          (the full source file for this example can be found here: <a href="../../../../../example/karma/complex_number_easier.cpp" target="_top">complex_number_easier.cpp</a>):
        </p>
<p>
</p>
<pre class="programlisting"><span class="keyword">template</span> <span class="special">&lt;</span><span class="keyword">typename</span> <span class="identifier">OutputIterator</span><span class="special">&gt;</span>
<span class="keyword">bool</span> <span class="identifier">generate_complex</span><span class="special">(</span><span class="identifier">OutputIterator</span> <span class="identifier">sink</span><span class="special">,</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">complex</span><span class="special">&lt;</span><span class="keyword">double</span><span class="special">&gt;</span> <span class="keyword">const</span><span class="special">&amp;</span> <span class="identifier">c</span><span class="special">)</span>
<span class="special">{</span>
    <span class="keyword">using</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">spirit</span><span class="special">::</span><span class="identifier">karma</span><span class="special">::</span><span class="identifier">double_</span><span class="special">;</span>
    <span class="keyword">using</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">spirit</span><span class="special">::</span><span class="identifier">karma</span><span class="special">::</span><span class="identifier">omit</span><span class="special">;</span>
    <span class="keyword">using</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">spirit</span><span class="special">::</span><span class="identifier">karma</span><span class="special">::</span><span class="identifier">generate</span><span class="special">;</span>

    <span class="keyword">return</span> <span class="identifier">generate</span><span class="special">(</span><span class="identifier">sink</span><span class="special">,</span>

        <span class="comment">//  Begin grammar</span>
        <span class="special">(</span>
           <span class="special">!</span><span class="identifier">double_</span><span class="special">(</span><span class="number">0.0</span><span class="special">)</span> <span class="special">&lt;&lt;</span> <span class="char">'('</span> <span class="special">&lt;&lt;</span> <span class="identifier">double_</span> <span class="special">&lt;&lt;</span> <span class="string">", "</span> <span class="special">&lt;&lt;</span> <span class="identifier">double_</span> <span class="special">&lt;&lt;</span> <span class="char">')'</span>
        <span class="special">|</span>   <span class="identifier">omit</span><span class="special">[</span><span class="identifier">double_</span><span class="special">]</span> <span class="special">&lt;&lt;</span> <span class="identifier">double_</span> <span class="special">&lt;&lt;</span> <span class="identifier">omit</span><span class="special">[</span><span class="identifier">double_</span><span class="special">]</span>
        <span class="special">),</span>
        <span class="comment">//  End grammar</span>

        <span class="identifier">c</span><span class="special">.</span><span class="identifier">imag</span><span class="special">(),</span> <span class="identifier">c</span><span class="special">.</span><span class="identifier">real</span><span class="special">(),</span> <span class="identifier">c</span><span class="special">.</span><span class="identifier">imag</span><span class="special">()</span>     <span class="comment">//  Data to output</span>
    <span class="special">);</span>
<span class="special">}</span>
</pre>
<p>
        </p>
<p>
          Let's cover some basic library features first.
        </p>
<h6>
<a name="spirit.karma.tutorials.karma_easier_complex.h0"></a>
          <span class="phrase"><a name="spirit.karma.tutorials.karma_easier_complex.making_numeric_generators_fail"></a></span><a class="link" href="karma_easier_complex.html#spirit.karma.tutorials.karma_easier_complex.making_numeric_generators_fail">Making
          Numeric Generators Fail</a>
        </h6>
<p>
          All <a class="link" href="../reference/numeric.html" title="Numeric Generators">Numeric Generators</a>
          (such as <code class="computeroutput"><span class="identifier">double_</span></code>, et.al.)
          take the value to emit from an attached attribute.
        </p>
<pre class="programlisting"><span class="keyword">double</span> <span class="identifier">d</span> <span class="special">=</span> <span class="number">1.5</span><span class="special">;</span>
<span class="identifier">generate</span><span class="special">(</span><span class="identifier">out</span><span class="special">,</span> <span class="identifier">double_</span><span class="special">,</span> <span class="identifier">d</span><span class="special">);</span>       <span class="comment">// will emit '1.5' (without the quotes)</span>
</pre>
<p>
          Alternatively, they may be initialized from a literal value. For instance,
          to emit a constant <code class="computeroutput"><span class="number">1.5</span></code> you
          may write:
        </p>
<pre class="programlisting"><span class="identifier">generate</span><span class="special">(</span><span class="identifier">out</span><span class="special">,</span> <span class="identifier">double_</span><span class="special">(</span><span class="number">1.5</span><span class="special">));</span>     <span class="comment">// will emit '1.5' as well (without the quotes)</span>
</pre>
<p>
          The difference to a simple <code class="computeroutput"><span class="number">1.5</span></code>
          or <code class="computeroutput"><span class="identifier">lit</span><span class="special">(</span><span class="number">1.5</span><span class="special">)</span></code> is that
          the <code class="computeroutput"><span class="identifier">double_</span><span class="special">(</span><span class="number">1.5</span><span class="special">)</span></code> consumes
          an attribute if one is available. Additionally, it compares its immediate
          value to the value of the supplied attribute, and fails if those are not
          equal.
        </p>
<pre class="programlisting"><span class="keyword">double</span> <span class="identifier">d</span> <span class="special">=</span> <span class="number">1.5</span><span class="special">;</span>
<span class="identifier">generate</span><span class="special">(</span><span class="identifier">out</span><span class="special">,</span> <span class="identifier">double_</span><span class="special">(</span><span class="number">1.5</span><span class="special">),</span> <span class="identifier">d</span><span class="special">);</span>  <span class="comment">// will emit '1.5' as long as d == 1.5</span>
</pre>
<p>
          This feature, namely to succeed generating only if the attribute matches
          the immediate value, enables numeric generators to be used to dynamically
          control the way output is generated.
        </p>
<div class="note"><table border="0" summary="Note">
<tr>
<td rowspan="2" align="center" valign="top" width="25"><img alt="[Note]" src="../../../images/note.png"></td>
<th align="left">Note</th>
</tr>
<tr><td align="left" valign="top"><p>
            Quite a few generators will fail if their immediate value is not equal
            to the supplied attribute. Among those are all <a class="link" href="../reference/char.html" title="Char Generators">Character
            Generators</a> and all <a class="link" href="../reference/string.html" title="String Generators">String
            Generators</a>. Generally, all generators having a sibling created
            by a variant of <code class="computeroutput"><span class="identifier">lit</span><span class="special">()</span></code>
            belong into this category.
          </p></td></tr>
</table></div>
<h6>
<a name="spirit.karma.tutorials.karma_easier_complex.h1"></a>
          <span class="phrase"><a name="spirit.karma.tutorials.karma_easier_complex.predicates___the_conditionals_for_output_generators"></a></span><a class="link" href="karma_easier_complex.html#spirit.karma.tutorials.karma_easier_complex.predicates___the_conditionals_for_output_generators">Predicates
          - The Conditionals for Output Generators</a>
        </h6>
<p>
          In addition to the <a class="link" href="../reference/auxiliary/eps.html" title="Epsilon Generator (eps)"><code class="computeroutput"><span class="identifier">eps</span></code></a> generator mentioned earlier
          <span class="emphasis"><em>Spirit.Karma</em></span> provides two special operators enabling
          dynamic flow control: the <a class="link" href="../reference/operator/and_predicate.html" title="And-Predicate Generator (&amp;a)">And
          predicate (unary <code class="computeroutput"><span class="special">&amp;</span></code>)</a>
          and the <a class="link" href="../reference/operator/not_predicate.html" title="Not-Predicate Generator (!a)">Not
          predicate (unary <code class="computeroutput"><span class="special">!</span></code>)</a>.
          The main property of both predicates is to discard all output emitted by
          the attached generator. This is equivalent to the behavior of predicates
          used for parsing. There the predicates do not consume any input allowing
          to look ahead in the input stream. In Karma, the and predicate succeeds
          as long as its associated generator succeeds, while the not predicate succeeds
          only if its associated generator fails.
        </p>
<div class="note"><table border="0" summary="Note">
<tr>
<td rowspan="2" align="center" valign="top" width="25"><img alt="[Note]" src="../../../images/note.png"></td>
<th align="left">Note</th>
</tr>
<tr><td align="left" valign="top"><p>
            The generator predicates in <span class="emphasis"><em>Spirit.Karma</em></span> consume
            an attribute, if available. This makes them behave differently from predicates
            in <span class="emphasis"><em>Spirit.Qi</em></span>, where they do not expose any attribute.
            This is because predicates allow to make decisions based on data available
            only at runtime. While in <span class="emphasis"><em>Spirit.Qi</em></span> during parsing
            the decision is made based on looking ahead a few more input tokens,
            in <span class="emphasis"><em>Spirit.Karma</em></span> the criteria has to be supplied
            by the user. The simplest way to do this is by providing an attribute.
          </p></td></tr>
</table></div>
<p>
          As an example, the following generator succeeds generating
        </p>
<pre class="programlisting"><span class="keyword">double</span> <span class="identifier">d</span> <span class="special">=</span> <span class="number">1.0</span><span class="special">;</span>
<span class="identifier">BOOST_ASSERT</span><span class="special">(</span><span class="identifier">generate</span><span class="special">(</span><span class="identifier">out</span><span class="special">,</span> <span class="special">&amp;</span><span class="identifier">double_</span><span class="special">(</span><span class="number">1.0</span><span class="special">),</span> <span class="identifier">d</span><span class="special">));</span>    <span class="comment">// succeeds as d == 1.0</span>
</pre>
<p>
          while this one will fail:
        </p>
<pre class="programlisting"><span class="keyword">double</span> <span class="identifier">d</span> <span class="special">=</span> <span class="number">1.0</span><span class="special">;</span>
<span class="identifier">BOOST_ASSERT</span><span class="special">(!</span><span class="identifier">generate</span><span class="special">(</span><span class="identifier">out</span><span class="special">,</span> <span class="special">!</span><span class="identifier">double_</span><span class="special">(</span><span class="number">1.0</span><span class="special">),</span> <span class="identifier">d</span><span class="special">));</span>   <span class="comment">// fails as d == 1.0</span>
</pre>
<p>
          Neither of these will emit any output. The predicates discard everything
          emitted by the generators to which they are applied.
        </p>
<h6>
<a name="spirit.karma.tutorials.karma_easier_complex.h2"></a>
          <span class="phrase"><a name="spirit.karma.tutorials.karma_easier_complex.ignoring_supplied_attributes"></a></span><a class="link" href="karma_easier_complex.html#spirit.karma.tutorials.karma_easier_complex.ignoring_supplied_attributes">Ignoring
          Supplied Attributes</a>
        </h6>
<p>
          Sometimes it is desirable to 'skip' (i.e. ignore) a provided attribute.
          This happens for instance in alternative generators, where some of the
          alternatives need to extract only part of the overall attribute passed
          to the alternative generator. <span class="emphasis"><em>Spirit.Karma</em></span> has a special
          pseudo generator for that: the directive <a class="link" href="../reference/directive/omit.html" title="Generator Directives Consuming Attributes (omit[] and skip[])"><code class="computeroutput"><span class="identifier">omit</span></code></a><code class="computeroutput"><span class="special">[]</span></code>.
          This directive consumes an attribute of the type defined by its embedded
          generator but it does not emit any output.
        </p>
<div class="note"><table border="0" summary="Note">
<tr>
<td rowspan="2" align="center" valign="top" width="25"><img alt="[Note]" src="../../../images/note.png"></td>
<th align="left">Note</th>
</tr>
<tr><td align="left" valign="top"><p>
            The <span class="emphasis"><em>Spirit.Karma</em></span> <a class="link" href="../reference/directive/omit.html" title="Generator Directives Consuming Attributes (omit[] and skip[])"><code class="computeroutput"><span class="identifier">omit</span></code></a> directive does the 'opposite'
            of the directive of the same name in <span class="emphasis"><em>Spirit.Qi</em></span>.
            While the <a class="link" href="../../qi/reference/directive/omit.html" title="Parser Directive Ignoring Attribute (omit[])"><code class="computeroutput"><span class="identifier">omit</span></code></a> in <span class="emphasis"><em>Spirit.Qi</em></span>
            consumes input without exposing an attribute, its <span class="emphasis"><em>Spirit.Karma</em></span>
            counterpart consumes an attribute without emitting any output.
          </p></td></tr>
</table></div>
<h6>
<a name="spirit.karma.tutorials.karma_easier_complex.h3"></a>
          <span class="phrase"><a name="spirit.karma.tutorials.karma_easier_complex.putting_everything_together"></a></span><a class="link" href="karma_easier_complex.html#spirit.karma.tutorials.karma_easier_complex.putting_everything_together">Putting
          everything together</a>
        </h6>
<p>
          Very similar to our first example earlier we use two alternatives to allow
          for the two different output formats depending on whether the imaginary
          part of the complex number is equal to zero or not. The first alternative
          is executed if the imaginary part is not zero, the second alternative otherwise.
          This time we make the decision during runtime using the <a class="link" href="../reference/operator/not_predicate.html" title="Not-Predicate Generator (!a)">Not
          predicate (unary <code class="computeroutput"><span class="special">!</span></code>)</a>
          combined with the feature of many Karma primitive generators to <span class="emphasis"><em>fail</em></span>
          under certain conditions. Here is the first alternative again for your
          reference:
        </p>
<pre class="programlisting"><span class="special">!</span><span class="identifier">double_</span><span class="special">(</span><span class="number">0.0</span><span class="special">)</span> <span class="special">&lt;&lt;</span> <span class="char">'('</span> <span class="special">&lt;&lt;</span> <span class="identifier">double_</span> <span class="special">&lt;&lt;</span> <span class="string">", "</span> <span class="special">&lt;&lt;</span> <span class="identifier">double_</span> <span class="special">&lt;&lt;</span> <span class="char">')'</span>
</pre>
<p>
          The generator <code class="computeroutput"><span class="special">!</span><span class="identifier">double_</span><span class="special">(</span><span class="number">0.0</span><span class="special">)</span></code>
          does several things. First, because of the <a class="link" href="../reference/operator/not_predicate.html" title="Not-Predicate Generator (!a)">Not
          predicate (unary <code class="computeroutput"><span class="special">!</span></code>)</a>,
          it succeeds only if the <code class="computeroutput"><span class="identifier">double_</span><span class="special">(</span><span class="number">0.0</span><span class="special">)</span></code>
          generator <span class="emphasis"><em>fails</em></span>, making the whole first alternative
          fail otherwise. Second, the <code class="computeroutput"><span class="identifier">double_</span><span class="special">(</span><span class="number">0.0</span><span class="special">)</span></code>
          generator succeeds only if the value of its attribute is equal to its immediate
          parameter (i.e. in this case <code class="computeroutput"><span class="number">0.0</span></code>).
          And third, the not predicate does not emit any output (regardless whether
          it succeeds or fails), discarding any possibly emitted output from the
          <code class="computeroutput"><span class="identifier">double_</span><span class="special">(</span><span class="number">0.0</span><span class="special">)</span></code>.
        </p>
<p>
          As we pass the imaginary part of the complex number as the attribute value
          for the <code class="computeroutput"><span class="special">!</span><span class="identifier">double_</span><span class="special">(</span><span class="number">0.0</span><span class="special">)</span></code>,
          the overall first alternative will be chosen only if it is not equal to
          zero (the <code class="computeroutput"><span class="special">!</span><span class="identifier">double_</span><span class="special">(</span><span class="number">0.0</span><span class="special">)</span></code>
          does not fail). That is exactly what we need!
        </p>
<p>
          Now, the second alternative has to emit the real part of the complex number
          only. In order to simplify the overall grammar we strive to unify the attribute
          types of all alternatives. As the attribute type exposed by the first alternative
          is <code class="computeroutput"><span class="identifier">tuple</span><span class="special">&lt;</span><span class="keyword">double</span><span class="special">,</span> <span class="keyword">double</span><span class="special">,</span> <span class="keyword">double</span><span class="special">&gt;</span></code>,
          we need to skip the first and last element of the attribute (remember,
          we pass the real part as the second attribute element). We achieve this
          by using the <code class="computeroutput"><span class="identifier">omit</span><span class="special">[]</span></code>
          directive:
        </p>
<pre class="programlisting"><span class="identifier">omit</span><span class="special">[</span><span class="identifier">double_</span><span class="special">]</span> <span class="special">&lt;&lt;</span> <span class="identifier">double_</span> <span class="special">&lt;&lt;</span> <span class="identifier">omit</span><span class="special">[</span><span class="identifier">double_</span><span class="special">]</span>
</pre>
<p>
          The overall attribute of this expression is <code class="computeroutput"><span class="identifier">tuple</span><span class="special">&lt;</span><span class="keyword">double</span><span class="special">,</span> <span class="keyword">double</span><span class="special">,</span> <span class="keyword">double</span><span class="special">&gt;</span></code>, but the <code class="computeroutput"><span class="identifier">omit</span><span class="special">[]</span></code> 'eats up' the first and the last element.
          The output emitted by this expression consist of a single generated double
          representing the second element of the tuple, i.e. the real part of our
          complex number.
        </p>
<div class="important"><table border="0" summary="Important">
<tr>
<td rowspan="2" align="center" valign="top" width="25"><img alt="[Important]" src="../../../images/important.png"></td>
<th align="left">Important</th>
</tr>
<tr><td align="left" valign="top"><p>
            Generally, it is preferable to use generator constructs not requiring
            semantic actions. The reason is that semantic actions often use constructs
            like: <code class="computeroutput"><span class="identifier">double_</span><span class="special">[</span><span class="identifier">_1</span> <span class="special">=</span> <span class="identifier">c</span><span class="special">.</span><span class="identifier">real</span><span class="special">()]</span></code>.
            But this assignment is a real one! The data is in fact <span class="emphasis"><em>copied</em></span>
            to the attribute value of the generator attached to the action. On the
            other hand, grammars without any semantic actions usually don't have
            to copy the attributes, making them more efficient.
          </p></td></tr>
</table></div>
</div>
<table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr>
<td align="left"></td>
<td align="right"><div class="copyright-footer">Copyright © 2001-2011 Joel de Guzman, Hartmut Kaiser<p>
        Distributed under the Boost Software License, Version 1.0. (See accompanying
        file LICENSE_1_0.txt or copy at <a href="http://www.boost.org/LICENSE_1_0.txt" target="_top">http://www.boost.org/LICENSE_1_0.txt</a>)
      </p>
</div></td>
</tr></table>
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